1. Field of the Invention:
Surveying of wellbore orientation is commonly performed by the use of instruments containing sets of three orthogonal accelerometers and magnetometers, which are inserted within the drillstring and used to measure the orientations of the local gravitational and magnetic field vectors. In order to measure the earth's magnetic field, which is used as a north reference from which wellbore azimuth may be computed, the instruments must be placed within a section of non-magnetic material extending between upper and lower ferromagnetic drillstring sections. These ferromagnetic portions of the drillstring tend to acquire magnetization as they are repeatedly strained in the earth's magnetic field during drilling operations. The nominally non-magnetic portion of the drillstring may also acquire some lesser magnetization as a result of imperfections. The result is that magnetometer measurements made by an instrument within a drillstring may measure not the undisturbed magnetic field, but the vector sum of the earth's field and an error field caused by drillstring magnetization. Since the tool is fixed with respect to the drillstring, the error field is fixed with respect to the tool's coordinate system and it appears as bias errors on the magnetometer measurements, which can lead to errors in the determination of wellbore azimuth and trajectory unless measures are taken to compensate for these bias errors.
2. Description of the Prior Art:
Since the greater part of the drillstring magnetization occurs in the ferromagnetic portions of the drillstring, which are displaced axially from the instrument, the bias error in the axial direction usually exceeds the transverse bias errors. Various methods have therefore been published which seek to determine axial magnetometer bias errors in a single directional survey, including U.S. Pat. Nos. 3,791,043 to Russell, 4,163,324 to Russell, U.S. Pat. No. Re. 33,708 to Roesler, 4,761,889 to Cobern, 4,819,336 to Russell, 4,999,920 to Russell, and 5,155,916 to Engebretson. All of these methods require the provision of an independent estimate of one or more components of the earth's magnetic field, and as a result all of them tend to lose accuracy in those attitudes in which the direction of the independent estimate is perpendicular to the drillstring and therefore contributes little or no axial information. In particular, all of these methods lose accuracy as the wellbore attitude approaches horizontal east-west. A number of methods have also been published which seek to determine magnetometer biases on all three axes, including U.S. Pat. Nos. 4,682,421 to van Dongen and 4,956,921 to Coles, and UK Pat. No. 2,256,492 to Nicolle. While certain of these methods can resolve transverse bias components without external estimates of the field, they all require an independent estimate of the earth's magnetic field in order to determine the axial bias component, and therefore they also tend to lose accuracy as the attitude approaches horizontal east-west. U.S. Pat. No. 4,709,486 to Walters discloses a method for determining axial bias errors without any external estimate, by the simultaneous use of transverse magnetometer data from a plurality of surveys. However the method fails to make use of the valuable information contained in the axial magnetometer measurements, since it does not require any correlation between the axial biases determined at the two locations. In U.S. Pat. No. 5,321,893, Engebretson discloses a method which may be used to determine magnetometer scale factor and bias errors from a plurality of surveys with or without requiring any external estimate of the earth's field. However, the method is inherently approximate since it requires the construction of a "measurement matrix", whose elements depend on the unknown borehole attitude and magnetic dip angle.
Additional objectives, features and advantages of the present invention will be apparent in the written description which follows.